The present invention relates to the generation of high frequency coherent electromagnetic radiation such as soft x-rays and, more particularly, to a device and method for producing a beam of this radiation.
The development of masers and lasers circa 1960 stimulated speculation about the possibility to develop devices that generate coherent electromagnetic radiation with much shorter wavelengths, for example, of soft x-ray wavelengths between about 1 nm and about 10 nm. Such devices could be used in applications including holographic imagine of biological structures, plasma diagnostics and the generation of intense plasmas. More recent advances in other fields have suggested other applications of coherent soft x-rays. For example, a source of coherent x-rays for CT scanning could be operated at a much lower power level than the sources of incoherent x-rays now in use, reducing the exposure of the subjects to ionizing radiation. Another application is in the fabrication of microdevices. The design rule of devices such as integrated circuits is now limited to a lower bound of about 0.1 microns by several factors, not least of which is that the shortest wavelength radiation that can be used for photolithography is ultraviolet radiation. A coherent source of soft x-rays would help make even shorter design rules feasible. Similarly, a coherent source of soft x-rays would enable much denser storage of information in media such as compact disks. A sufficiently intense coherent beam of soft x-rays can be used as an industrial cutting tool.
Lasers have been used to produce soft x-rays. The first soft x-ray laser was developed at Lawrence Livermore National Laboratory in 1984. This device, which was described in general terms by Dennis Matthews and Mordecai Rosen in the December 1988 issue of Scientific American, uses the Nova laser to create a plasma including high-Z ions and to create a population inversion among those ions by collisional excitation. The laser medium in such a device is inherently transient. Essentially, the mere process of creating the laser medium, by evaporating a metal foil to produce plasma, also destroys the laser medium. The use of the Nova laser, originally developed for controlled fusion research, to create the lacer medium also meant that the device was a large, expensive research tool unsuitable for practical applications. More compact soft x-ray lasers have been developed in recent years, but like their giant ancestor at LLNL, they all rely on inherently self-destructive mechanisms to create a population inversion in a highly ionized plasma.
Two non-self-destructive strategies for the generation of coherent soft x-rays have been explored. The first is the imposition of spatial periodicity on the trajectories of high-energy electrons in free-electron lasers. This requires the use of a massive high-energy accelerator to create the high-energy electrons.
The second non-self-destructive strategy for the generation of coherent soft x-rays is the use of frequency multiplication media to create higher harmonics of coherent light. This strategy has led to the development of devices that produces high frequency coherent electromagnetic beams from low frequency coherent electromagnetic beams produced by conventional lasers. A review of the strategy may be found in L""Huiller et al., xe2x80x9cHigh-order harmonics: a coherent source in the XUV rangexe2x80x9d, Journal of Nonlinear Optical Physics and Materials, July 1995, Vol. 4 No. 3, pp. 647-665, which is incorporated by reference for all purposes as if fully set forth herein.
Using helium atoms as the frequency multiplication medium, Chang et al. in Phys. Rev. Lett. 79, pp. 2967-2970 (1997) have reported the generation of beams with down to 2.7 nm wavelengths while Schnxc3xcrer et al. in Phys. Rev. Lett. 80, pp. 3236-3239 (1998) have reported the generation of beams with wavelengths down to shorter than 2.5 nm.
WO 99/33084, which is incorporated by reference for all purposes as if fully set forth herein, discloses a device where a substantially circularly polarized beam of coherent light is directed at a frequency multiplication medium. The frequency multiplication medium of WO 99/33084 is made up of a material, having constituents, the constituents having an approximate Nth order rotational symmetry axis, oriented so that the Nth order axes of the constituents are substantially parallel to the incident beam. The interaction of the beam and the constituents produces selected higher frequency harmonics of the frequency of the incident beam. If the Nth order rotational symmetry axis is exact, the harmonic frequencies produced are integral multiples of the frequency of the harmonic beam, given by xcexa9=Nkxc2x11, where k is a positive integer. If the Nth order rotational axis symmetry is only approximate, the harmonics are centered around these multiples. xe2x80x9cApproximate Nth order rotational symmetry axisxe2x80x9d means that the geometry of the constituents of the frequency multiplication medium is close enough to having an exact Nth order rotational symmetry axis for the interaction of the frequency multiplication medium with the incident beam to lead to frequency multiplication in accordance with the approximate selection rules. Depending on N and on the frequency of the incident beam, the harmonics produced are up into the soft x-ray band. Suitable medium constituents include molecules with a dipole and C5 symmetry (such as C5H5Tl gas) circular rings of nanoparticles and nanotubes.
The methods and devices known in the art are insufficient because ordinarily N is not very large, generally N=2 for atoms in monochromatic linearly polarized fields, leading to the emission of many harmonics and thus a broad mixture of wavelengths. For example, N. Sarukura et al. reported in Phys. Rev. A Vol 43 pp. 1669-1672 (1991) the generation of 9th to 23rd order harmonics of light from a KrF laser when using helium as a frequency multiplication medium. Preston et al in Phys. Rev. A Vol. 53 pp. R31-R34 (1996) reported obtaining harmonics up to the 35th using helium as a frequency multiplication medium.
As is clear to one skilled in the art, for most applications it is preferred that the distribution of wavelengths in the produced beam be as limited as possible.
There is thus a widely recognized need for, and it would be highly advantageous to have, a compact, portable, reusable source of coherent x-rays with a limited wavelength distribution.
According to the present invention there is provided a device for producing high frequency radiation, including: a) a source of elliptically polarized radiation; and b) a frequency multiplication medium including at least one constituent having approximate finite symmetry including a screw axis of approximate Nth order symmetry, wherein N is at least 2, and wherein the constituent is oriented so that the radiation propagates substantially parallel to the screw axis.
According to the present invention there is also provided a method of producing high frequency radiation, including the steps of: a) providing a frequency multiplication medium including at least one constituent having approximate finite symmetry including a screw axis of approximate Nth order symmetry, wherein N is at least 2; and b) directing elliptically polarized radiation at an angle to the screw axis so that the elliptically polarized radiation propagates substantially parallel to the screw axis
According to the present invention there is provided a device for producing high frequency radiation, including: a) a source of elliptically polarized radiation; and b) a frequency multiplication medium including at least one constituent having approximate finite symmetry including a screw axis of approximate Nth order symmetry, wherein N is at least 2 and also including an axis of approximate Cn symmetry, wherein n is at least 3, and wherein the constituent is oriented so that the radiation propagates substantially parallel to the screw axis.
According to the present invention there is also provided a method of producing high frequency radiation, including the steps of: a) providing a frequency multiplication medium including at least one constituent having approximate finite symmetry including a screw axis of approximate Nth order symmetry, wherein N is at least 2 and also including an axis of approximate Cn symmetry, wherein n is at least 3; and b) directing elliptically polarized radiation at an angle to the screw axis so that the elliptically polarized radiation propagates substantially parallel to the screw axis.
According to the present invention there is also provided a method of producing a beam of high frequency radiation composed of a limited number of wavelengths by: a) providing a frequency multiplication medium including at least one constituent having approximate finite symmetry including a screw axis of approximate Nth order symmetry, wherein N is at least 2; and b) directing elliptically polarized radiation at an angle to the axis such that the elliptically polarized radiation propagates substantially parallel to the screw axis. The intensity of the radiation is chosen relative to the frequency of the radiation, the magnitude of N and the geometry of the constituent so that only the desired number of harmonics is produced.